Actually, Jimmy wasn't supposed to exercise along the dark side of the crater wall. The grown ups said it was dangerous, but that was because they were never there. The ground was smooth and crunchy and Jimmy knew the exact location of every one of the few rocks.
Besides, how could it be dangerous racing through the dark when Robutt was right there with him, bouncing around and squeaking and glowing? Even without the glow, Robutt could tell where he was, and where Jimmy was, by radar. Jimmy couldn't go wrong while Robutt was around, tripping him when he was too near a rock, or jumping on him to show how much he loved him, or circling around and squeaking low and scared when Jimmy hid behind a rock, when all the time Robutt knew well enough where he was. Once Jimmy had lain still and pretended he was hurt and Robutt had sounded the radio alarm and people from Lunar City got there in a hurry. Jimmy's father had let him hear about that little trick, and Jimmy never tried it again.
Just as he was remembering that, he heard his father's voice on his private wavelength. "Jimmy, come back. I have something to tell you."